1. Field of the Invention
This invention relates to a photoconductor coupled liquid-crystal light valve utilizing photoconductive effect of a photoconductive material and electro-optical effect of liquid crystals, and a preparation of the same.
2. Description of the Related Art
In recent years, as a memory device for optical information, a photoconductor coupled liquid-crystal light valve utilizing photoconductive effect of a photoconductive material and electrooptical effect of liquid crystals has been developed. FIG. 3 shows a basic construction of a photoconductor coupled liquid crystal light valve. The photoconductor coupled liquid crystal light valve comprises photoconductive layers 1, 1-, a liquid crystal layer 2, transparent electrodes 3, 3 sandwiching the photoconductive layers 1, 1 and the liquid crystal layer 2, a voltage applying means 4 provided with the transparent electrodes 3, 3, a reflection layer 5 interposing between one of the photoconductive layers and the liquid crystal layer, orientation films 6, 6 and glass substrates 7,7. Basic function of this device will be detailed hereunder. For convenience of explanation, the resistance value of the reflection layer 5 and orientation film 6 is assumed to be substantially lower than that of the photoconductive layer and the liquid crystal layer.
When voltage V.sub.0 is applied to the photoconductive layer 1 and the liquid crystal layer 2 by the voltage applying means 4, the photoconductive layer 1 and the liquid-crystal layer 2 are imposed with the voltages in proportion to resistance of the layers 1 and 2, respectively. When light is not applied to the photoconductive layer 1, since resistance of the photoconductive layer is high, the voltage applied to the liquid-crystal layer 2 is lower than the threshold voltage VST that enables electrooptical effect. When light 8 for writing is applied in a predetermined ON/OFF pattern from the side of the photoconductive layer 1, the irradiated part of the photoconductive layer 1 decreases in resistance to make higher voltage to be applied to the corresponding part of the liquid crystal layer. In this instance, the voltage exceeds the threshold voltage V.sub.ST to cause a change of the layer due to electro-optical effect. The remaining part of the photoconductive layer not subjected to the irradiation does not change in resistance, so that there causes no change in the liquid crystal layer. This is the process of writing of optical information. When light is applied from the side of the liquid crystal layer 2 after the above writing of optical information, the reflection layer 5 provides a reading light 9 with a pattern of existence and non-existence of reflective light.
Such a device operation as above requires to satisfy the following formula to show a fine contrast and store a good image. EQU R.sub.PCt &lt;&lt;R.sub.LC &lt;&lt;R.sub.PCd (1)
wherein R.sub.PCt is resistance in the irradiated part, R.sub.LC is resistance of the liquid crystal layer, and R.sub.PCd is resistance in the non-irradiated part. Assuming that the photoconductive layer 1 and the liquid crystal layer 2 have a substantially equal thickness to each other, the formula (1) can be converted into a formula with specific resistance .rho.(.OMEGA.cm) as follows. EQU .rho..sub.PCt &lt;&lt;.rho..sub.LC &lt;&lt;.rho..sub.PCd (2)
To be especially appreciated in the formulas (1) and (2) is to increase R.sub.PCd and .rho..sub.PCd.
The use of amorphous silicon (called hereunder a-Si) as the above photoconductive layer has been disclosed, for example, in Japanese Unexamined Patent Publications Nos.58-34435(1983), 58-34436(1983), 58-199327(1983), 59-81627(1984) and 59-170820(1984) wherein a-Si is prepared by plasma CVD process or sputtering process, and the following methods have been used for increasing R.sub.PCd and .rho..sub.PCd.
I. Boron is added to the a-Si layer to make it an intrinsic semiconductor having a high .rho..sub.PCd.
II. A metallic material for forming a Schottky contact with respect to the photoconductive layer is used in a translucent form as a transparent electrode to be in contact with the photoconductive layer for making higher nominal specific resistance of the a-Si layer, to which the transparent electrode is applied an inverse bias to substantially increase .rho..sub.PCd.
III. The thickness of a-Si layer is made larger to increase R.sub.PCd.
The photoconductor coupled light valve prepared by the above manners I to III has the following problems.
I. Even when boron is doped, .rho..sub.PCd is about 10.sup.10 to 10.sup.11 .OMEGA.cm and the selective range of liquid crystal is limited to those of specific resistance .rho..sub.LC= 10.sup.11 .OMEGA.cm or lower. Also, various documentations show that when boron is doped to photoconductive layer, its photosensitivity is deteriorated, so that liquid crystal layer corresponding to the part subjected to irradiation with light is hard to be applied with sufficient voltage, leading to low in switching speed (for providing electro-optic effect) and causing no switching of liquid crystal layer, in turn, no operation of device.
II. The current excited by the photoirradiation becomes hard to flow and nominal photosensitivity is deteriorated, leading to the problems as referred to in the item I. Also, the electrode is made of a metallic material for forming Schottky contact, so that the light-transmittance lowers to affect the property of the optical information writing device. Furthermore, such a device is to be used only in direct current mode for maintaining Schottky contact.
III. Conventionally, plasma CVD process was extensively used for the deposition of the a-Si layer. Depositing speed in the process is very slow, so that it takes longer time for formation of thicker deposition, leading to a raised cost.
Also, deposition according to plasma CVD process or sputtering process allows the production of an polymeric powder of (SiH).sub.n which adheres to a surface of substrate in the process of deposition to create defects in the deposited layer.
The present invention has been accomplished to overcome the above problems. An object of the invention is to provide a photoconductor coupled liquid crystal light valve of good property with a wider selective range of liquid crystal, by improving specific resistance of the photoconductive layer. An another object of the invention is to provide a photoconductor coupled liquid crystal light valve and a preparation of the same enabling prevention of an increase of cost to produce of the device.
The preparation of an amorphous silicon layer according to electron cyclotron resonance process (ECR process) has been disclosed, for example, in U.S. Pat. No. 4532199. However, any disclosure on the application of the amorphous silicon layer prepared by ECR process for a photoconductive layer in the photoconductor coupled light valve is not at all known in the art.